The present application discloses systems and methods to fast start high pressure heat recovery steam generators (HRSGs) to minimize thermal stress failures experienced by conventional HRSGs. Thousands of Combined Cycle (CC) power plants have been installed and are operating throughout the world. In first thirty years most were installed as base load and designed to be started only a few dozen times a year. Operating experience on many proved that start-stops initiated the greatest damage obliging major repairs, replacement and reduced availability in only a few hundred start cycles for many sites. Increasing the requirement to start faster and more times a year on higher pressure combined cycles necessitates innovative systems and methods focused on solving this proven industry problem. At present CC are often specified to cycle daily and even hourly to load follow and start in 30 minutes (to accommodate solar and wind power variability). To match advances in gas turbine technology with resultant high exhaust temperatures, steam pressure, and steam temperature has been increased to improve efficiency, power density and reduce specific costs of power. These requirements thereby result in increasing thermal stresses and mechanical problems for new HRSGs designs still basically using the same designs as the past conventional HRSG configurations. Improvements have been incorporated in some units through incorporation of the once-through Benson design that eliminates the thick wall high pressure drum a practical necessity as steam pressures move ever higher. However, large HRSG designs still incorporate thousands small diameter vertical thin wall finned tubes (20 to 30 meters long) connected at the top and bottom to thick wall headers in 70 to 90 rows. Often two or three rows are welded into a single header rigidly constraining each tube ends. The Lower headers are free to expand downward to accommodate the average expansion of two or more rows of tubes welded across the length of the header. This highly rigid design has proven to be acceptable for base load even though tube rows have different temperatures in the same header. During fast starts higher temperature differentials exist between tube rows, tubes in the same row and thick wall headers. After a number of start cycles the differential temperatures result in failure of weld joints and tubes. The use of bypass exhaust stacks are necessary in many installations to allow gradual warm up to prevent failure but are not fast starters and result in efficiency loss. This is a problem is especially acute during fast starts and shutdowns of high pressure HRSGs even with the Benson HRSG that only solves the thick wall high pressure drum problem by eliminating it entirely.
Thermal shock during starts is a result of rapid differential radial and lengthwise tube expansion eventually causing numerous weld joint cracks, tube cracks and tube buckling with the top and bottom header configurations installed in conventional units. Particularly vulnerable are the high pressure superheater and reheater header to tube joints and tubes. During the start sequence the thin wall tubes in the high pressure superheater and reheater rapidly heat much faster than the headers. After ignition in the combustion gas turbine little or no steam flows in the superheater or reheater to cool the tubes that rapidly heat to approach the exhaust gas temperature in the first few minutes. The thick wall headers metal temperature remains at a lower temperature taking 600% longer than the superheater and reheater tubes to approach operating temperature. This severe lag occurs because the headers temperature rise is determined by steam flow steam initiated from the downstream thermally massive evaporator section of the HRSG. Differential temperature of several hundred degrees between the tubes and headers eventually results in fatigue cracking of tube to header welds. Rapid tube length expansion and different tube temperatures row to row is constrained by the upper and lower rigid headers bending stresses on the joint and often to buckled tubes.
Cold starts cause the highest fatigue damage, to minimize damage in cycling duty the HRSG is bottled up overnight and kept at as high a steam pressure possible. When started the superheater has stagnant saturated steam that condenses in the tubes during the start ramp. Condensate forms as a result of motoring the gas turbine for about ten minutes to circulate cold air to purge ducting and the HRSG of possible explosive gases prior to ignition. Water condensate from the saturated steam in the tubes collects in the lower headers and interconnecting pipes and has proven difficult to drain prior to the initiation of steam flow. In many units as steam flow is initiated many minutes later the condensate water is stripped out of the lower headers and forced downstream unevenly across the rows of the now very hot superheater tubing quenching tubes unevenly. As a result some tubes go into tension and other tubes buckle and the fatigue life of tube to header weld joints is reduced in some units to only a few hundred start cycles. The invention method eliminates the lower headers and fills the tubes with saturated water eliminating this problem.
Another condensate cause of thermal strain is shutdown from high power (trips). During turbine trips compressor spin-down circulates cold exhaust air flow through the superheater and reheater tubing producing condensate as the tube metal temperature is reduced to the saturation temperature of the steam in the evaporator, condensate hundreds of degrees colder than the hot header metal quenching the hot lower headers and pipes. Cracking of lower tube weld joints, header tube bores and distortion of headers and pipes can result from this thermal shock quenching particularly with the high temperatures required for new high pressure CC power plants. No viable solutions to this damage have been obtained with the conventional HRSGs. The invention eliminates the thick wall lower headers and replaces them with thin wall tubes formed in U-bends.
Another source of water quenching damage is interstage attemperation in the high pressure superheater and reheater. During starts the conventional HRSGs require attemperation water sprays into the thick wall interconnecting piping between superheater and reheater headers to prevent over temperature during the start when steam flow is low and gas temperature is high. Spraying water into these pipes is another major observed cause of thermal shock damage in many units. Overspray is a common and difficult to prevent control problem due to lag times and lack of long strait pipe lengths in the interstage pipes between stages. During starts and other operating modes water spray often impinges on pipe wall quenching them, is then stripped from walls quenching headers and tubes downstream causing cracks.